1. Field of the Invention
This invention relates to a titanium oxide-filled addition reaction-curable silicone rubber composition having a low viscosity and low thixotropic properties.
2. Description of the Prior Art
Titanium oxide is conventionally added in silicone compositions as a colorant. As its new uses, e.g., there is a demand for uses as a light-screening agent in thin films or conversely as a reflecting agent. Stated specifically, there is a demand for its use in parallel type reflecting agents of photocoupler devices and reflective undercoatings of LEDs.
To meet such a demand, it is necessary to add the titanium oxide in a large quantity. However, adding titanium oxide in a silicone rubber composition in a large quantity as conventionally done makes the composition have a high viscosity and high thixotropic properties to make it difficult to handle and also provide no satisfactory operability.
Accordingly, an object of the present invention is to provide a titanium oxide-filled addition reaction-curable silicone rubber composition having a low viscosity and low thixotropic properties and easy to handle, even though it contains titanium oxide in a large quantity, and especially having a superior operability when used as a light-screening agent or a reflecting agent.
To achieve the above object, the present invention provides a titanium oxide-filled addition reaction-curable silicone rubber composition comprising;
(A) a diorganopolysiloxane containing at least two alkenyl groups in one molecule;
(B) an organohydrogenpolysiloxane containing in one molecule at least two hydrogen atoms bonded to silicon atoms;
(C) a platinum group metal catalyst; and
(D) a titanium oxide powder having been surface-treated with a compound selected from the group consisting of a tetraalkoxysilane and a partial hydrolysis-condensation product of a tetraalkoxysilane; the titanium oxide powder being contained in an amount not less than 15 parts by weight based on 100 parts by weight of the component (A).
The present invention also provides a cured product obtained by curing the above composition.
The present invention will be described below in detail.
(A) Alkenyl Group-containing Diorganopolysiloxane:
The component-(A) diorganopolysiloxane is a compound containing at least two alkenyl groups in one molecule, and is used as a base polymer of the composition of the present invention. This alkenyl group-containing diorganopolysiloxane is normally a substantially linear (or straight-chain) compound whose backbone chain moiety consists basically of repetition of diorganopolysiloxane units and both molecular-chain terminals are terminated with triorganopolysiloxyl groups. It may also contain a branched structure at some parts of its molecular structure, or may have a cyclic structure on the whole. In particular, in view of physical properties such as mechanical strength of cured products, straight-chain diorganopolysiloxanes are preferred. The alkenyl group may be present only on the both terminals of the molecular chain, may be present only in the course of the molecular chain as side-chain substituents, or may be present both on the both terminals of the molecular chain and in the course of the molecular chain.
As typical examples of such an alkenyl group-containing diorganopolysiloxane, it may include diorganopolysiloxanes represented by the following general formula (1): 
wherein R1""s each independently represent a substituted or unsubstituted monovalent hydrocarbon group containing no aliphatic unsaturated bond, X represents an alkenyl group, Y""s each represent an alkenyl group or a group represented by R1, n is an integer of 0 or 1 or more, and m is an integer of 0 or 1 or more; having in the molecule at least two alkenyl groups bonded to silicon atoms.
In the general formula (1), the substituted or unsubstituted monovalent hydrocarbon group containing no aliphatic unsaturated bond, represented by R1 may include, e.g., alkyl groups such as a methyl group, a ethyl group, a propyl group, a isopropyl group, a butyl group, a isobutyl group, a tert-butyl group, a pentyl group, a neopentyl group, a hexyl group, a heptyl group, a octyl group, a nonyl group, a decyl group and a dodecyl group; cycloalkyl groups such as a cyclopentyl group, a cyclohexyl group and a cycloheptyl group; aryl groups such as a phenyl group, a tolyl group, a xylyl group, a naphthyl group and a biphenyl group; aralkyl groups such as a benzyl group, a phenylethyl group, a phenylpropyl group and a methylbenzyl group; and any of these groups at least part of hydrogen atoms bonded to carbon atoms of which has been substituted with a halogen atom such as fluorine, chlorine or bromine or a cyano group, as exemplified by halogen-substituted alkyl groups, cyano-substituted alkyl groups and halogen-substituted aryl groups such as a chloromethyl group, a 2-bromomethyl group, a 3-chloropropyl group, a 3,3,3-trifluoropropyl group, a chlorophenyl group, a fluorophenyl group, a cyanoethyl group, a 3,3,4,4,5,5,6,6,6-nonafluorohexyl group. Typical groups represented by R1 are those having 1 to 10 carbon atoms, in particular, those having 1 to 6 carbon atoms, and preferably substituted or unsubstituted alkyl groups having 1 to 3 carbon atoms, such as a methyl group, an ethyl group, a propyl group, a chloromethyl group, a bromoethyl group, a 3,3,3-trifluorpropyl group and a cyanoethyl group; and substituted or unsubstituted phenyl groups such as a phenyl group, a chlorophenyl group and a fluorophenyl group.
In the general formula (1), the alkenyl group represented by X may usually include those having 2 to 8 carbon atoms as exemplified by a vinyl group, an allyl group, a propenyl group, a isopropenyl group, a butenyl group, a hexenyl group and a cyclohexenyl group. In particular, lower alkenyl groups such as a vinyl group and an allyl group are preferred.
In the general formula (1), n is an integer of 0 or more, and m is an integer of 0 or more. Also, n and m may preferably be integers that satisfy 10xe2x89xa6n+mxe2x89xa610,000 and 0xe2x89xa6m/ (m+n)xe2x89xa60.2, and particularly preferably integers that satisfy 50xe2x89xa6n+mxe2x89xa62,000 and 0xe2x89xa6m/(m+n) xe2x89xa60.05. Y""s each represent an alkenyl group or the group represented by R1. This alkenyl group may include the same as those exemplified for those of X, and R1 is as defined above, provided that Y""s as substituents bonded to silicon atoms at the molecular-chain both terminals may both preferably be alkenyl groups.
This alkenyl group-containing diorganopolysiloxane may also preferably be those having a viscosity at 25xc2x0 C. of from 1 to 100,000 poises, and particularly from about 10 to 50,000 poises.
(B) Organohydrogenpolysiloxane:
The component-(B) is a compound having in one molecule at least two, and preferably at least three, hydrogen atoms bonded to silicon atoms (i.e., SiH groups), and is used as a cross-linking agent (D). This organohydrogenpolysiloxane may be any of resinous products having a straight-chain, branched, cyclic or three-dimensional network structure. As typical examples of such an organohydrogenpolysiloxane, it may include, e.g., organohydrogenpolysiloxanes represented by the following average compositional formula (2):
HaR2 bSiO(4-a-b)/2 xe2x80x83xe2x80x83(2)
wherein R2 independently represents a substituted or unsubstituted monovalent hydrocarbon group containing no aliphatic unsaturated bond; and a and b are numbers that satisfy 0 less than a less than 2, 0.7xe2x89xa6bxe2x89xa62 and 0.8xe2x89xa6a+bxe2x89xa63, and preferably 0.001xe2x89xa6axe2x89xa61.2, 0.8xe2x89xa6bxe2x89xa62 and 1xe2x89xa6a+bxe2x89xa62.7, and more preferably 0.01xe2x89xa6axe2x89xa61, 1.2xe2x89xa6bxe2x89xa62 and 1.8xe2x89xa6a+bxe2x89xa62.4.
In the average compositional formula (2), the substituted or unsubstituted monovalent hydrocarbon group containing no aliphatic unsaturated bond, represented by R2 may include the same as those exemplified for R1 in the general formula (1). Typical groups represented by R2 are those having 1 to 10 carbon atoms, in particular, those having 1 to 7 carbon atoms, and may preferably be lower alkyl groups having 1 to 3 carbon atoms, such as a methyl group; a phenyl group; and a 3,3,3-trifluoropropyl group.
Such organohydrogenpolysiloxanes may specifically include, e.g., siloxane oligomers such as 1,1,3,3-tetramethyldisiloxane, 1,3,5,7-tetramethyltetracyclosiloxane and 1,3,5,7,8-pentamethylpentacyclosiloxane; siloxane polymers such as methylhydrogenpolysiloxanes terminated with trimethylsiloxyl groups at both terminals of the molecular chain, dimethylsiloxane-methylhydrogensiloxane copolymers terminated with trimethylsiloxyl groups at both terminals of the molecular chain, methylhydrogenpolysiloxanes terminated with silanol groups at both terminals of the molecular chain, dimethylsiloxane-methylhydrogensiloxane copolymers terminated with silanol groups at both terminals of the molecular chain, dimethylpolysiloxanes terminated with dimethylhydrogensiloxyl groups at both terminals of the molecular chain, methylhydrogenpolysiloxanes terminated with dimethylhydrogensiloxyl groups at both terminals of the molecular chain, dimethylsiloxane-methylhydrogensiloxane copolymers terminated with dimethylhydrogensiloxyl groups at both terminals of the molecular chain, dimethylsiloxane-diphenylsiloxane-methylhydrogensiloxane copolymers terminated with trimethylsiloxyl groups at both terminals of the molecular chain, and dimethylsiloxane-diphenylsiloxane-methylhydrogensiloxane copolymers terminated with trimethylhydrogensiloxyl groups at both terminals of the molecular chain; and silicone resins which consist of an R2(H)SiO1/2 unit and an SiO4/2 unit and may optionally contain an R3SiO1/2 unit, an R2SiO2/2 unit, an R(H)SiO2/2 unit, an (H)SiO3/2 unit or an RSiO3/2 unit (in the formulas, R""s are the same as those for the substituted or unsubstituted monovalent hydrocarbon group exemplified as R1). They may further include those represented by the following formulas: 
(k is an integer of 1 to 200.) 
(m is an integer of 0 to 100, and n is an integer of 1 to 100.) 
(p is an integer of 0to 100, and q is an integer of 2 to 100, and preferably an integer of 3 to 100.) 
(m and q are as defined above, and r is an integer of 1 to 50.) 
(m, n and r are as defined above.) 
(m, q and r are as defined above.) 
(m, n and r are as defined above.) 
(m, q and r are as defined above.) 
(m, n and r are as defined above.)
As this component-(B) organohydrogenpolysiloxane, those having a viscosity at 25xc2x0 C. of usually from 0.002 to 10 poises, and particularly from 0.005 to 5 poises, may be used.
The component-(B) organohydrogenpolysiloxane can be produced by any known processes. It may be produced by, e.g., co-hydrolysis of at least one chlorosilane selected from the group consisting of those represented by the following general formulas (3) and (4):
R2SiHCl2xe2x80x83xe2x80x83(3)
R22SiHClxe2x80x83xe2x80x83(4)
wherein R2 is as defined in the average compositional formula (2); or an alkoxyl derivative thereof (e.g., a methoxyl derivative), or co-hydrolysis of this chlorosilane or alkoxyl derivative thereof (e.g., a methoxyl derivative) together with at least one chlorosilane selected from the group consisting of those represented by the following general formulas (5) and (6):
R23SiClxe2x80x83xe2x80x83(5)
R22SiCl2 xe2x80x83xe2x80x83(6)
wherein R2 is as defined in the average compositional formula (2). The organohydrogenpolysiloxane may also be produced by subjecting to equilibrium reaction the polysiloxane obtained by hydrolysis carried out in such a way.
The component (B) may be used in such an mount that hydrogen atoms bonded to silicon atoms (i.e., SiH groups) in the component-(B) organohydrogenpolysiloxane are present usually in an amount of from 0.5 to 8 moles, and preferably from 1 to 5 moles, per mole of the alkenyl groups in the component-(A) alkenyl group-containing diorganopolysiloxane.
(C) Platinum Group Metal Catalyst:
The component-(C) platinum group metal catalyst is a catalyst for accelerating the addition reaction (hydrosililation reaction) of the component-(A) alkenyl group-containing diorganopolysiloxane with the SiH groups of the component-(B). As the platinum group metal catalyst, any well known catalysts for hydrosililation reaction may be used. As specific examples thereof, they may include, e.g., platinum group metal simple substances such as platinum (inclusive of platinum black), rhodium and palladium; platinum chloride, chloroplatinic acid and chloroplatinates such as H2PtCl4. nH2O, H2PtCl6. nH2O, NaHPtCl6. nH2O, KHPtCl6. nH2O, Na2PtCl6. nH2O, K2PtCl4. nH2O, PtCl2 and Na2HPtCl4. nH2O (wherein n is an integer of 0 to 6, and preferably 0 or 6); alcohol-modified chloroplatinic acid (see U.S. Pat. No. 3,220,972); complexes of chloroplatinic acid with olefins (see U.S. Pat. Nos. 3,159,601, 3,159,662 and 3,775,452); catalysts comprising a platinum group metal such as platinum black or palladium supported on a support such as alumina, silica or carbon; rhodium-olefin complexes; chlorotris(triphenylphosphine)rhodium (a Wilkinson catalyst); and complexes of platinum chloride, chloroplatinic acid or chloroplatinates with vinyl-group-containing siloxanes, in particular, vinyl-group-containing cyclic siloxanes.
The component (C) may be used in what is called a catalytically effective quantity, and may usually be in an mount of from about 0.1 to 1,000 ppm, and particularly from about 0.5 to 500 ppm, in terms of the weight of the platinum group metal, based on the total weight of the components (A) and (B).
(D) Titanium Oxide Powder Having been Surface-treated with Tetraalkoxysilane and/or Partial Hydrolysis-condensation Product of Tetraalkoxysilane:
The component-(D) surface-treated titanium oxide powder is a fine titanium oxide powder having been surface-treated with at least one compound selected from the group consisting of a tetraalkoxysilane and a partial hydrolysis-condensation product of tetraalkoxysilane, and is a component that chiefly colors cured products (in white), endows them with light-screening properties, reflecting properties, heat resistance, mechanical properties and so forth, or reinforces such properties.
(i) Tetraalkoxysilane and/or Partial Hydrolysis-condensation Product of Tetraalkoxysilane:
The tetraalkoxysilane used in the surface treatment of titanium oxide powder is represented by the following general formula (7):
Si(OR3)4 xe2x80x83xe2x80x83(7)
wherein R3""s each independently represent a lower alkoxyl-substituted or unsubstituted alkyl group.
In the general formula (7), the lower alkoxyl-substituted or unsubstituted alkyl group represented by R3 may include, e.g., those usually having about 1 to 12 carbon atoms, and preferably about 1 to 6 carbon atoms, such as a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a t-butyl group, a pentyl group, a neopentyl group, a hexyl group, a heptyl group, a octyl group, a nonyl group, a decyl group, a dodecyl group, a methoxymethyl group, a methoxyethyl group, an ethoxymethyl group and an ethoxyethyl group.
Such a partial hydrolysis-condensation product of tetraalkoxysilane may be a siloxane condensation product having at least one, and preferably at least two, residual alkoxyl group(s) in one molecule, and may include those represented by the following average compositional formula (8):
[OR3)3SiOl/2]K[OR3)2SiO2/2]L[OR3)SiO3/2]M[SiO4/2]Nxe2x80x83xe2x80x83(8)
wherein R3 is as defined previously; K, L, M and N are numbers that satisfy K+L+M+N=1, 0.002xe2x89xa6Kxe2x89xa61, 0xe2x89xa6Lxe2x89xa60.998, 0xe2x89xa6Mxe2x89xa60.998 and 0xe2x89xa6Nxe2x89xa60.35.
In particular, it may typically include those represented by the following general formula (9):
R3Oxe2x80x94[xe2x80x94Si(OR3)2Oxe2x80x94]jxe2x80x94R3xe2x80x83xe2x80x83(9)
wherein R3 is as defined previously, and j is an integer of 2 or more, preferably an integer of 2 to 1,000, and more preferably an integer of 2 to 200.
Such a partial hydrolysis-condensation product of tetraalkoxysilane may preferably have a weight-average molecular weight in terms of polystyrene, of from 150 to 100,000, and more preferably from 200 to 20,000.
As a surface-treating agent therefor, one or more types of tetraalkoxysilanes may be used alone, or one or more types of partial hydrolysis-condensation products of tetraalkoxysilane may be used alone. Alternatively, the tetraalkoxysilane and the partial hydrolysis-condensation product of tetraalkoxysilane may be used in combination in any desired proportion.
(ii) Titanium Oxide:
The titanium oxide powder used for the surface treatment may be any of those known in the art. It commonly includes two types, rutile and anatase. Especially when used in light-screening agents or reflecting agents of electronic component parts, those not surface-treated with any aluminum or organic matter are preferred. It may usually have an average particle diameter of from about 0.05 to 10 xcexcm, and preferably from about 0.1 to 5 xcexcm, which may appropriately selected in accordance with uses. Incidentally, the average particle diameter may usually be determined as a value of weight-average particle diameter (or median diameter) by, e.g., measurement with an electron microscope or laser light diffraction.
As methods for the surface treatment, any known wet-process treatment or dry-process treatment may be used. As a specific example of surface treatment methods, a method is available in which the tetraalkoxysilane and/or the partial hydrolysis-condensation product of tetraalkoxysilane is/are dissolved or dispersed in a suitable solvent and thereafter the titanium oxide powder is mixed in the resultant solution or dispersion, followed by heating and drying. The solvent may include toluene and xylene. The heating and drying may be carried out, e.g., at 80 to 200xc2x0 C. for about 0.5 to 10 hours, which may be processed under reduced pressure in combination.
As another surface treatment method, a method is available in which, e.g., at least part (usually 30% by weight or more, and particularly 50% by weight or more) of the alkenyl-group containing diorganopolysiloxane as the component (A), the titanium oxide powder and the tetraalkoxysilane and/or the partial hydrolysis-condensation product of tetraalkoxysilane are mixed and the resultant mixture is heat-treated. Theheattreatmentmaybemadeunderconditionsof,e.g., 80 to 200xc2x0 C. and about 0.5 to 10 hours. The method in which the surface treatment is made using such a mixture of at least part of the component (A), the titanium oxide powder and the tetraalkoxysilane and/or the partial hydrolysis-condensation product of tetraalkoxysilane is preferred because the titanium oxide powder can be surface-treated simultaneously in the step of mixing or kneading when as described later the components (A) to (D) and optionally other additive components are mixed or kneaded to produce the composition of the present invention and hence the rationalization attributable to energy saving and a shorter process time can be achieved.
The tetraalkoxysilane and/or the partial hydrolysis-condensation product of tetraalkoxysilane may be used in an amount appropriately adjusted in accordance with specific surface area and other properties of the titanium oxide powder, and may usually be in an amount of from 0.1 to 20 parts by weight, and preferably from 0.5 to 10 parts by weight, based on 100 parts by weight of the titanium oxide powder. As a result of such surface treatment, films of siloxane are formed on the particle surfaces of the titanium oxide powder, so that its wettability (or affinity) to silicone resin components is improved and a composition having a low viscosity and low thixotropic properties and rich in fluidity can be obtained.
The component-(D) surface-treated titanium oxide powder may usually be in an amount of 15 parts by weight or more, preferably from 15 to 300 parts by weight, more preferably from 20 to 250 parts by weight, and particularly preferably from 30 to 200 parts by weight, based on 100 parts by weight of the component-(A) alkenyl group-containing diorganopolysiloxane. If it is in a too small amount, the resultant composition may have a low effect on light-screening or reflection.
Other Component:
In addition to the components (A) to (D), any additives usually used may be added to the composition of the present invention. Such additives may include, e.g., reinforcing inorganic fillers such as fumed silica, precipitated silica and fumed titanium dioxide; and non-reinforcing inorganic fillers such as crushed silica, fused silica, crystalline silica (quartz powder), calcium silicate, ferric oxide and carbon black. Any of these inorganic fillers may usually be added in an amount of form 0 to 200 parts by weight based on 100 parts by weight of the total weight of the components (A) to (D).
Especially when the composition is used in a two-pack type as described later, a cure control agent may also be added which includes acetylene alcohol or silane-modified derivatives or siloxane-modified derivatives of acetylene alcohol.
For the purpose of improving adhesion of the composition, an epoxy-group-containing polysiloxane compound or an ester siloxane compound may further be added in addition to the above components.
Silicone Rubber Composition, Production of its Cured Product, and Uses:
The composition of the present invention is produced basically by mixing or kneading the components (A) to (D) and optionally the additive components. In such a case, like conventional addition reaction-curable silicone rubber compositions, the components (A) to (D) may be divided into two parts as exemplified by xe2x80x98a part of component (A) and the component (D)xe2x80x99 and xe2x80x98the remaining component (A) and the component (B)xe2x80x99 to prepare what is called a two-part type composition the two parts of which are mixed and cured when used. Other two-part type composition may include a composition in which at least a part of the component (A), the titanium oxide powder and the tetraalkoxysilane and/or the partial hydrolysis-condensation product of tetraalkoxysilane have been mixed or kneaded when the titanium oxide powder is surface-treated and the resultant mixture or kneaded product has been heat-treated to prepare one part, and the remaining components have been mixed or kneaded to prepare another part. Alternatively, the components (A) to (D) may be mixed or kneaded together with a cure control agent added in a small quantity, to prepare what is called a one-part type composition.
The cured product of the present invention is obtained by curing such a silicone rubbercomposition. The composition maybe curedunder conditions like those for known addition reaction-curable silicone rubber compositions. For example, even at room temperature it cures sufficiently. It may optionally be heated.
The cured product of the present invention thus obtained has high light-screening properties and reflecting properties and also has a low viscosity and low thixotropic properties. Accordingly, it can be utilized in coating agents for light-screening and reflection in electric and electronic component parts.